This invention relates to removing water from an emulsion of oil and water and in particular to a voltage control system for controlling the voltage applied to the primary winding of a step-up transformer to limit the power available to the primary winding of the step-up transformer and hence the power available to the dehydration process so as not to exceed rated power of the step-up transformer.
A reactive impedance has been used in separation processes to protect transformers from exceeding rated power. The reactive impedance was placed in series with the primary winding of the transformer being protected and was designed to be a 100% reactive impedance, that is, with a short circuit on the secondary winding of the transformer, the current passing through the primary winding and reactive impedance was limited to rate current. Although this method of protecting a transformer was very effective, during normal operation the series reactive impedance limited the power delivered to the load on the secondary winding to the transformer to about one-half of the rated power of the transformer. Therefore, an oversized transformer was necessarily required for all applications utilizing a series, 100% reactive impedance as a means of protecting the transformer. Oversizing a transformer by a factor of 2 results in the capability to deliver the desired power to the separation process when a 100% reactive impedance is used to protect the transformer but makes the original equipment investment very expensive. The investment is even greater when the transformer is a high voltage, power transformer like those used in electrostatic precipitator and oil dehydrator applications.
An alternate method of protecting the transformer from exceeding rated power is to reduce or eliminate the series reactive impedance and place a voltage control circuit in series with the primary winding of the transformer. Usually a smaller reactive impedance remains in the circuit to filter current surges. Voltage control has been used in electrostatic precipitator separation processes. In adapting a voltage controller from an electrostatic precipitator separation process to an oil dehydration separation process, a problem was encountered that severely limited the effectiveness of the dehydration process. To understand the problem it is beneficial to first review the electrostatic precipitator separation process control application.
In the electrostatic precipitator application, a voltage controller was successful in controlling the power delivered to the primary winding of a step-up transformer so that the transformer rated power was not exceeded when conditions in the electrostatic precipitator were such that the electric field intensity exceeded the voltage at which the dielectric broke down and arcing occurred. A dielectric breakdown was detected by an undervoltage limit. The control system was developed to break the arc by detecting the arcing condition, reducing the voltage applied to the primary winding of the step-up transformer to zero for at least one-half cycle, then reapplying the voltage to the primary winding of the step-up transformer at a voltage level below the voltage level at which the arcing occurred.
When the air between grid elements ionized such that a conductive path was formed therebetween, the automatic voltage control system would go into a current limit condition. The conductive path of ionized air was essentially a short circuit across the secondary winding of the step-up transformer; the current limit protected the step-up transformer from exceeding rated power. During the current limit conditions, the separation process ceased, although throughflow and power consumption continued. No control action was taken to eliminate the cause of the current limit and thereby eliminate unnecssary power consumption as well as increase the ineffectiveness of the separation process.
Similar conditions arose in the electrostatic dehydrator when adapting a voltage controller from an electrostatic precipitator application. A dielectric breakdown occurred when the electric field intensity exceeded the voltage at which the dielectric broke down and arcing occurred. A dielectric breakdown was detected by an undervoltage limit. The automatic voltage control system responded as it responded to a dielectric breakdown in the electrostatic precipitator application.
When a conductive path of water formed between opposite polarity grid elements causing a low resistance path therebetween, the automatic voltage control system would go into a current limit condition. The low resistance path was essentially a short circuit across the secondary winding of the step-up transformer causing the dehydration process to cease although throughflow and power consumption continued. The low resistance path problem is different from the dielectric breakdown problem in that the dielectric breakdown will result in the automatic voltage control system limiting power by a voltage limit while a conductive path between opposite polarity grid elements result in the automatic voltage control system limiting power by a current limit.
There is a need for a means of protecting the step-up transformer in an electrostatic separation process from exceeding rated power during the occurrence of a low resistance path between opposite polarity grid elements that permits sizing the step-up transformer more consistently with the load requirements and upon the detection of a low resistance path between opposite polarity grid elements creating conditions favorable for breaking the low resistance path.